The present invention relates to a ceramic sintered body applicable to members required to have an electrically insultaing property, and subjected to the repetition of rapid temperature change while joined to metallic members, such as a heater supporting member for a glow plug.
Sintered bodies of silicon nitride(Si.sub.3 N.sub.4), aluminum nitride(AlN), .beta.-sialon[Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (z=0.about.4.2)] (hereinafter will be called "sialon"), or the like exhibit low thermal expansion coefficient and accordingly, they have been employed in such uses as to be required to have high thermal shock resistance.
However, it is well known that when these sintered bodies are jointed to metallic members, or when metallic members are embedded in these sintered bodies, thermal stress is formed due to the difference in thermal expansion coefficient between the sintered bodies and the metallic members to generate such heat fatigue in the sintered bodies as to be cracked.
For example, when a sintered body of Si.sub.3 N.sub.4 is used as the heater supporting member for a ceramic glow plug, the supporting member is cracked in its joined portions with a metallic housing which covers the supporting member and with a tungsten wire embedded therein.
Furthermore, in the case where the sintered bodies of different thermal expansion coefficients are joined to each other, they are liable to be cracked.
For preventing such cracks of the ceramic sintered bodies, conventionally, the thermal expansiion coefficient thereof has been controlled by dispersing Al.sub.2 O.sub.3 of which the thermal expansion coefficient is higher than that of the basic material of the ceramic bodies in these ceramic sintered bodies. However, in the above described conventional method, Al.sub.2 O.sub.3 dissolves in Si.sub.3 N.sub.4 in solid phase when sintered to change physical properties of the ceramic sintered bodies. And as the dissolution of Al.sub.2 O.sub.3 in solid phase proceeds, the amount of Al.sub.2 O.sub.3 as high thermal expansion coefficient material is decreased so that thermal expansion coefficient of the obtained sintered body is not so increased as to be expected.